1. Field of the Invention
The invention relates to a fine screen in the form of a coated metal fabric, and to a fine screen stack and its use, and it also includes a process for the manufacture of fine screens.
A fine screen in the context of the present description is a coated metal fabric which, depending on the duration of treatment and the intended application, has fine to extremely fine openings, down to molecular dimensions.
2. Description of the Prior Art
Metal fabrics are characterized by good mechanical strength and resistance to high temperatures and, when appropriate materials are used, by corrosion resistance and high efficiency with a defined "porosity" and a low pressure loss. They are therefore widely used as fine screen filters in various sectors, such as the chemical and petrochemical industries, nuclear technology, in pharmaceutical processes and pollution control, in the production of foods and beverages and in the aviation and space industry.
A significant disadvantage of the metal fabrics, however, consists in the relatively large mesh width, even with fine fabrics, which can retain only particles with a diameter of more than about 5 microns, while the particle sizes of the impurities in gases and/or fluids to be separated can be smaller than that by orders of magnitude.
Of course, with considerable effort and expense, the production of metal fabric has recently been refined (e.g. by Fuji in Japan), and mesh widths of approximately 2 microns are now feasible, although the market price of this fine fabric is approximately 4 to 5 times higher than that of the 5 micron fabric.
For the deposition of such extremely fine particles, therefore, when possible, fiber felts are used with extremely fine deposition active fibers, the filtering action of which is determined more by the large surface area of a tangle of extremely fine fibers than the effective mesh width, which is the governing factor for the pressure loss.
In simulation of such extremely fine filters manufactured on the basis of textiles or ceramics, fine metal fabrics are provided on one side with a sintered metal powder layer (such as "Supramesh" by the firm of Pall Process Filtration Limited), which retain particles with a size greater than 1.5 microns nominal or greater than 15 microns absolute from fluids, and particles of more than 0.5 microns nominal and 3 microns absolute from gases. Even such porous coated metal fabrics will not adequately retain the finest particles, even though the pressure loss caused by such materials is clearly greater than that of uncoated fabrics.